1. Field of the Invention
The invention relates to a device for monitoring a predetermined level in a container, including a sensor formed by a mechanical vibratory system comprising two vibratory rods, of which at least one vibratory rod is tubular and surrounding the other vibratory rod coaxially, each of the two vibratory rods being secured to a common support via a resilient holding member acting as a return spring so that each rod is able to execute vibrations transversely to its longitudinal direction, an excitation arrangement causing the two vibratory rods to vibrate in opposite senses at the natural resonant frequency of the mechanical vibratory system, and an evaluation circuit for triggering display or switching actions as a function of the vibration amplitude of the mechanical vibratory system of the sensor.
2. Description of the Prior Art
A device of this kind is known from U.S. Pat. No. 4,499,765. Each vibratory rod forms with the resilient holding member acting as a return spring a mechanical vibratory assembly, the natural resonant frequency of which is dictated by the mass moment of inertia of the vibratory rod and the spring constant of the resilient holding member. The two mechanical vibratory assemblies are configured so that they have the same natural resonant frequency which is simultaneously the natural resonant frequency of the mechanical vibratory system as a whole. For a given excitation power their opposed vibrations then have a maximum vibration amplitude when the outer vibratory rod vibrates in air, whereas when the outer vibratory rod is covered by the material, the level of which is to be monitored, the vibrations of the mechanical vibratory system are damped so that their amplitude becomes smaller or the vibration even collapses altogether. Due to the differing vibration amplitudes the evaluation circuit is thus able to detect whether the material has attained the level to be monitored or not.
In known devices of this kind there is the problem that the natural resonant frequency of the mechanical vibratory assembly formed by the outer vibratory rod is altered when a deposit of the material forms on the outer vibratory rod, since this results in the mass moment of inertia of the outer vibratory rod becoming larger, whereas the natural resonant frequency of the mechanical vibratory assembly formed by the inner vibratory rod remains unaltered. The two mechanical vibratory assemblies are then no longer tuned to each other, resulting in the vibration amplitude of the mechanical vibratory system becoming smaller. There is then the risk that the evaluation circuit is unable to recognize whether the reduction in the vibration amplitude is due to a deposit having been formed, although the outer vibratory rod is vibrating in air, or is due to the outer vibratory rod being covered by the material. This can result in false indications.